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>Chapter 46. Frontend/Backend Protocol</TD
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><H1
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><A
NAME="PROTOCOL-OVERVIEW"
>46.1. Overview</A
></H1
><P
>   The protocol has separate phases for startup and normal operation.
   In the startup phase, the frontend opens a connection to the server
   and authenticates itself to the satisfaction of the server.  (This might
   involve a single message, or multiple messages depending on the
   authentication method being used.)  If all goes well, the server then sends
   status information to the frontend, and finally enters normal operation.
   Except for the initial startup-request message, this part of the
   protocol is driven by the server.
  </P
><P
>   During normal operation, the frontend sends queries and
   other commands to the backend, and the backend sends back query results
   and other responses.  There are a few cases (such as <TT
CLASS="COMMAND"
>NOTIFY</TT
>)
   wherein the
   backend will send unsolicited messages, but for the most part this portion
   of a session is driven by frontend requests.
  </P
><P
>   Termination of the session is normally by frontend choice, but can be
   forced by the backend in certain cases.  In any case, when the backend
   closes the connection, it will roll back any open (incomplete) transaction
   before exiting.
  </P
><P
>   Within normal operation, SQL commands can be executed through either of
   two sub-protocols.  In the <SPAN
CLASS="QUOTE"
>"simple query"</SPAN
> protocol, the frontend
   just sends a textual query string, which is parsed and immediately
   executed by the backend.  In the <SPAN
CLASS="QUOTE"
>"extended query"</SPAN
> protocol,
   processing of queries is separated into multiple steps: parsing,
   binding of parameter values, and execution.  This offers flexibility
   and performance benefits, at the cost of extra complexity.
  </P
><P
>   Normal operation has additional sub-protocols for special operations
   such as <TT
CLASS="COMMAND"
>COPY</TT
>.
  </P
><DIV
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><H2
CLASS="SECT2"
><A
NAME="PROTOCOL-MESSAGE-CONCEPTS"
>46.1.1. Messaging Overview</A
></H2
><P
>   All communication is through a stream of messages.  The first byte of a
   message identifies the message type, and the next four bytes specify the
   length of the rest of the message (this length count includes itself, but
   not the message-type byte).  The remaining contents of the message are
   determined by the message type.  For historical reasons, the very first
   message sent by the client (the startup message) has no initial
   message-type byte.
  </P
><P
>   To avoid losing synchronization with the message stream, both servers and
   clients typically read an entire message into a buffer (using the byte
   count) before attempting to process its contents.  This allows easy
   recovery if an error is detected while processing the contents.  In
   extreme situations (such as not having enough memory to buffer the
   message), the receiver can use the byte count to determine how much
   input to skip before it resumes reading messages.
  </P
><P
>   Conversely, both servers and clients must take care never to send an
   incomplete message.  This is commonly done by marshaling the entire message
   in a buffer before beginning to send it.  If a communications failure
   occurs partway through sending or receiving a message, the only sensible
   response is to abandon the connection, since there is little hope of
   recovering message-boundary synchronization.
  </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PROTOCOL-QUERY-CONCEPTS"
>46.1.2. Extended Query Overview</A
></H2
><P
>    In the extended-query protocol, execution of SQL commands is divided
    into multiple steps.  The state retained between steps is represented
    by two types of objects: <I
CLASS="FIRSTTERM"
>prepared statements</I
> and
    <I
CLASS="FIRSTTERM"
>portals</I
>.  A prepared statement represents the result of
    parsing, semantic analysis, and (optionally) planning of a textual query
    string.
    A prepared statement is not necessarily ready to execute, because it might
    lack specific values for <I
CLASS="FIRSTTERM"
>parameters</I
>.  A portal represents
    a ready-to-execute or already-partially-executed statement, with any
    missing parameter values filled in.  (For <TT
CLASS="COMMAND"
>SELECT</TT
> statements,
    a portal is equivalent to an open cursor, but we choose to use a different
    term since cursors don't handle non-<TT
CLASS="COMMAND"
>SELECT</TT
> statements.)
   </P
><P
>    The overall execution cycle consists of a <I
CLASS="FIRSTTERM"
>parse</I
> step,
    which creates a prepared statement from a textual query string; a
    <I
CLASS="FIRSTTERM"
>bind</I
> step, which creates a portal given a prepared
    statement and values for any needed parameters; and an
    <I
CLASS="FIRSTTERM"
>execute</I
> step that runs a portal's query.  In the case of
    a query that returns rows (<TT
CLASS="COMMAND"
>SELECT</TT
>, <TT
CLASS="COMMAND"
>SHOW</TT
>, etc),
    the execute step can be told to fetch only
    a limited number of rows, so that multiple execute steps might be needed
    to complete the operation.
   </P
><P
>    The backend can keep track of multiple prepared statements and portals
    (but note that these exist only within a session, and are never shared
    across sessions).  Existing prepared statements and portals are
    referenced by names assigned when they were created.  In addition,
    an <SPAN
CLASS="QUOTE"
>"unnamed"</SPAN
> prepared statement and portal exist.  Although these
    behave largely the same as named objects, operations on them are optimized
    for the case of executing a query only once and then discarding it,
    whereas operations on named objects are optimized on the expectation
    of multiple uses.
   </P
></DIV
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><H2
CLASS="SECT2"
><A
NAME="PROTOCOL-FORMAT-CODES"
>46.1.3. Formats and Format Codes</A
></H2
><P
>    Data of a particular data type might be transmitted in any of several
    different <I
CLASS="FIRSTTERM"
>formats</I
>.  As of <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 7.4
    the only supported formats are <SPAN
CLASS="QUOTE"
>"text"</SPAN
> and <SPAN
CLASS="QUOTE"
>"binary"</SPAN
>,
    but the protocol makes provision for future extensions.  The desired
    format for any value is specified by a <I
CLASS="FIRSTTERM"
>format code</I
>.
    Clients can specify a format code for each transmitted parameter value
    and for each column of a query result.  Text has format code zero,
    binary has format code one, and all other format codes are reserved
    for future definition.
   </P
><P
>    The text representation of values is whatever strings are produced
    and accepted by the input/output conversion functions for the
    particular data type.  In the transmitted representation, there is
    no trailing null character; the frontend must add one to received
    values if it wants to process them as C strings.
    (The text format does not allow embedded nulls, by the way.)
   </P
><P
>    Binary representations for integers use network byte order (most
    significant byte first).  For other data types consult the documentation
    or source code to learn about the binary representation.  Keep in mind
    that binary representations for complex data types might change across
    server versions; the text format is usually the more portable choice.
   </P
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